5- and 12-lipoxygenases isolated from porcine leukocytes were investigated by electron paramagnetic resonance at X-band and atomic absorption spectroscopy. For comparison potato 5-lipoxygenase was studied under identical experimental conditions. All three lipoxygenases contained between 0.7 and 0.9 Fe atoms/enzyme molecule. As isolated, both mammalian enzymes exhibited a characteristic EPR signal at low magnetic field with a maximum at g = 5.20 indicative of a high-spin ferric iron center. The signal was not affected by the oxidants 12-hydroperoxyeicosatetraenoic acid or arachidonic acid, nor was it affected by the reductant nordihydroguaiaretic acid. In the case of the potato enzyme an intense EPR signal with resonances at g = 7.50, 6.39 and 5.84 was only observed after addition of an oxidant, such as 9-hydroperoxyoctadecadienoic acid.